I took the following from another site, its in two parts and I left the copyright request in as you will see. This is the best article I have found on the subject and will be more accurate than most of the advise you will get on BMW forums. I will do two posts as it is long. Read it, I think you might be in for a surprise or two:
Someone in the octane thread asked for a chemist's view on the subject, so here's mine. However, first a view necessities and background:
Ph.D. Organic chemist, 21 years as a practising chemist, 28 US patents (many on gasoline additves), work for a major oil company that is a leading (by volume) retailer of gasoline in the world. I've had engine tests (meaning an engine on a fixed engine stand run by a computer) and road tests (meaning cars driven by humans -- some on a course, some not) run on gasoline additives I've invented (while employed at the oil company). These additives have been tested in the US, England, and France under a variety of conditions. Many of my additives have also been tested in the BMW test (10,000 miles). I'm still employed by said oil company.
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Mine is a 1979 that is on its second trip through the odometer (and its second engine, and third interior). I've owned it approximately 12 years, love it, and may never get rid of it.
I've seen many consummer magazines tell their readers that they are wasting money to buy a higher octane gasoline if their car doesn't knock. If octane were not related to any other feature of the gasoline, I would agree. However, how companies get higher octane gasoline does make it different than regular and mid-grade gasoline. So the question to me then is, do these differences make a difference? The short answer is what do I use, and I use regular in all my cars and my truck. I use premium in my Corvette. Why? Because as any Corvette enthusiast (to separate us from people who merely own one -- no flames, please) knows, a Corvette is not a car, it's an experience. If you want to know the long answer keep reading.
Crude oil differs depending on where it's from, and consequently, what comes out of it when it's refined also differs. There are books on the subject and I can't possibly do the subject justice here. As the oil differs, what is available for blending and cutting is different. In the end, however, the refiner has to make economic choices, based on what's available to him from the oil he is refining, on how to get the required octane to sell (as well as meet MANY other criteria that make gasoline, gasoline -- again I can't do justice to it here). What I generally (but not exclusively) see is that BASE (no additve added to it -- you can't buy this, it isn't offered for sale) premium gasoline leaves less deposits behind than other grades. There are certain types of molecules in regular and mid-grade BASE gasolines that simply do not exist in premium gasoline that cause much of this. Additives (see definition below) are added to the gasoline to help get rid of these deposits, and modern additives do a marvelous job of this. What you buy is additized gasoline.
Blending components vs additives:
MTBE (methyl tertiary butyl ether -- this is ether) and ethanol are blending components, not additives. However, you'll see them called additives in many places by many people. Blending components are part of base gasoline. The simplest differentiation between an additve and a blending component is that the former are added in ppm (parts-per-million) levels; whereas, the latter are added in percentage quantities.
Brand of gasoline:
Some of you probably know this, others may be shocked by it. Oil companies swap base gasoline all the time. Let's say I have a refinery in Houston and you have one in Dallas. It makes little sense for me to truck my Houston gasoline to Dallas and for you to truck your gasoline from Dallas to Houston when gasoline is a commodity product. So, I let you draw 100,000 gallons of base gasoline from my storage tank in Houston for your Houston gas stations, and you let me draw 100,000 gallons of base gasoline from your Dallas holding tanks for my Dallas gas stations. That way, we both save on shipping. Yup, Texaco gasoline may have come from a Shell refinery and vice-versa. At a gasoline terminal you may see trucks from up to six different companies all loading at the SAME terminal (that for example may be supplied exclusively by Shell). What comes next, however, is what makes Texaco Texaco and Shell Shell. Additive. Each company has its own additive and adds it to the base gasoline. So while the base gasoline may be the same, the additive is different, and hence the brand of gasoline you use is different because of the additive, not the base gasoline.
Which additive is better?:
Given the above discourse, it's obvious that we all want the gasoline with the best additive. Unfortunately, it's not that simple. Additves respond differently in different base gasolines (even of the same grade). Also, some additves work better with gasolines used in a carbureted car vs one that uses fuel injection. On a practical level, additives are going to be developed today for today's cars -- meaning fuel injected cars. For Corvette owners who have carbureted Vettes (like me), this is unfortunate. Carbureted engines leave a LOT more deposits behind than fuel injected cars. From a regulatory stand point, California was the first to call for all gasolines to pass the BMW test (port-fuel-injected engine) in all grades of gasoline. Like all regulations, this one had various massages put to it, but the net effect was that all oil companies went to work developing additives that are a LOT better today than 10 years ago AND they are used in all grades (not just premium -- hence the argument to use premium to get a better additive went out the window).
Insider's trick on gasoline additives:
No matter what you do or what you drive, this trick will help you keep down deposits inside your engine. You see, additives themselves will make deposits and/or create a deposit that is different from the one made by base gasoline alone. If you think about this for a moment, you'll come to realize that your engine will build some kind of deposit based upon what additive you are using. Yes, it will build at a slower rate, but it will build deposits. At some level this will taper off (but this is maximum deposits and what Corvette owner wants that!). So what do you do? Simple, switch to a different brand of gasoline (this will almost assure you of getting a different additive but not always. Some companies buy additives from other companies, so it could be the same. More on this later). What this will do, is the new additve will look at the deposit formed from the old additive as foreign and begin removing it. Now after 5000 miles, you'll be rid of this deposit but you'll have a new one from your most recent additive, so switch back and start the process all over again. As an analogy, this is like building an immune response to an anti-biotic, so your doctor gives you a new one. I know of absolutely no additive that will work as well as switching back and forth between additives. On a molecular level this makes perfect sense.
So what's a Vette owner to do?:
(1) Whether you buy regular, mid-grade, or premium, use one brand for about 5000 miles, then switch to another brand for 5000 miles. I use Shell, then Chevron, then Exxon, then back to Shell, etc. (this is not an endorsement). These companies have historically had there own additive research groups/companies, so they'll likely use their own additive and not something they bought from each other. It is completely posible, however, to use a sequence like Amoco, Shell, Texaco and still come out fine. (Again these are not endorsements). I use three companies and 5000 miles based on what I've observed working in the research area. 7,000 miles and two different brands will still do wonders. As an aside, it takes most vehicles 7-13,000 miles to build their maximum deposit levels.
(2) If your Vette is carbureted or TBI, buy premium. These fuel-delivery systems build deposits rapidly and to much much higher levels than PFI (port fuel injection) systems You need every edge you can get (if you're an enthusiast). If I had a PFI Vette I'd still buy premium, but must tell you the effect will not be like in the carbureted version.